In order for a refrigeration unit, such as a transport refrigeration unit, to efficiently maintain a conditioned space and its associated load in a predetermined temperature range close to a selected set point temperature, it is important that air distribution throughout the conditioned space be proper. The refrigeration unit, via an evaporator fan or blower, discharges conditioned air into the conditioned space from a discharge air outlet associated with an evaporator plenum. Air is drawn from the conditioned space by the evaporator fan or blower back into the evaporator plenum via a return air inlet.
The discharge air should circulate throughout the conditioned space so that heat exchange between the conditioned air and load is as uniform as possible throughout the entire load, before the air is allowed to return to the air inlet. To insure uniform, highly efficient heat transfer between the conditioned air and load, chutes and/or bulkheads are often installed in the conditioned space. If conditioned air is allowed to return prematurely to the evaporator plenum, a condition which is called "short cycling", the heat transfer efficiency suffers, and the load in the conditioned space will not be uniformly conditioned. Short cycling may be caused, for example, by improper installation of air chutes and/or bulk heads in the conditioned space.
Refrigeration unit monitoring and diagnostic programs of the prior art may, upon encountering short cycling, incorrectly indicate a fault with the refrigeration unit, identifying low refrigeration capacity as the cause. It would be desirable, and it is an object of the present invention, to provide monitoring and diagnostic methods which differentiate between a short cycling air flow problem and a refrigeration capacity problem.